Ethanol from plant cultivation represents a possibility for a renewable energy and is suited as a fuel for operating internal combustion engines using the Otto cycle. Combustion engines, which are operated in the so-called flex-fuel mode, may be operated using a conventional hydrocarbon compound, e.g., gasoline, as well as alcohol, e.g., ethanol, or using any mixture of these two fuels. However, in this connection, a software functionality of a control unit for controlling the combustion engine must be adapted to a current mixture ratio of the alcohol (ethanol) and the hydrocarbon compound (gasoline). The alcohol concentration of a utilized fuel mixture must be known for this, since several operational characteristics maps of the engine control unit depend on it. As a rule, an ethanol sensor is used for determining the ethanol concentration.
German Patent Application No. DE 10 2008 002 493 A1 describes a method and a device for determining the composition of a fuel mixture made up of a first and at least one second fuel for operating an internal combustion engine, the combustion engine having a fuel metering device and at least one exhaust gas analyzer probe in an exhaust duct. In this connection, it is provided that the composition of the fuel mixture be determined from a signal for a pump current or for a change in pump current of a pump cell of the exhaust gas analyzer probe, the signal being a function of the components of the exhaust gas. In this context, the exhaust gas analyzer probe is configured as a wide-range lambda probe, which is situated close to the engine, in front of a first catalytic converter in the direction of the flow of exhaust gas. The composition of a fuel mixture is used in a combustion engine operated in flex-fuel mode. The use of a wide-range lambda probe allows additional sensors to be dispensed with, which produces cost advantages.
A further method for determining the composition of a fuel mixture is described in German Patent Application No. DE 10 2008 040 798 A1. In this connection, a first and at least one second fuel are used for operating an internal combustion engine, the combustion engine being controlled by an engine control unit, and the composition of the fuel mixture being determined by a fuel composition sensor. The fuel mixtures of varying composition have different specific heat capacities and/or different thermal conductivities. It is provided that a temperature sensor having a temperature-dependent electrical resistance be used as a fuel composition sensor, and that the fuel mixture be supplied to the temperature sensor. In a first measuring phase, the temperature of the fuel mixture is determined by the temperature sensor. In a second measuring phase, the temperature sensor is acted upon by a heating current and heated. In a third measuring phase, the heating current is switched off, and, from the time characteristic of the electrical resistance of the temperature sensor or a characteristic quantity derived from it during the second and/or third measuring phase, the composition of the fuel mixture is determined from its temperature and flow velocity at the temperature sensor.
A similar method for determining the composition of a fuel mixture is described in German Patent Application No. DE 10 2007 054 813 A1. In this, a heating element is provided for preheating and/or for evaporating the fuel mixture. In this case, use is made of the fact that fuel mixtures of varying composition have different specific heat capacities and/or different heats of vaporization and/or different boiling points. In this context, it is provided that the fuel mixture be fed to the heating element, and that the heat energy removed from the heating element by the fuel mixture, or a characteristic quantity dependent on it, be determined. The composition of the fuel mixture is determined from a heat energy removed from the heating element, or from the characteristic quantity dependent on it. The method allows the composition of a fuel mixture in an internal combustion engine operated in flex-fuel mode to be determined in an accurate and reliable manner.
German Patent Application No. DE 10 2007 023 899 A1 also describes a method for determining the composition of a fuel mixture made up of a first and at least one second fuel for operating an internal combustion engine having at least one combustion chamber; it also being taken into account that the first and the at least one second fuel have different boiling points and/or different enthalpies of vaporization. In this case, the composition of the fuel mixture is ascertained from the pressure in the combustion chamber and/or from a characteristic quantity linked to the pressure and/or from the time characteristic of the pressure and/or from the time characteristic of a characteristic quantity linked to the pressure, during and/or after an injection of fuel during a compression phase of the fuel-air mixture.